1. Technical Field
The disclosed embodiments relate in general to a display, and more particularly to a high resolution organic electroluminescence display (OLED).
2. Description of the Related Art
In comparison to the conventional cathode-ray tube display device, the flat panel display device, such as the liquid crystal display device (LCD), the plasma display device (PDP) and the organic electroluminescent display (OELD) device), are lighter, slimmer and compacter, and has become the mainstream product. The OELD device, in particular, has advantages of light weight, flexibility, easy to carry (portability), full color, high brightness, lower power consumption, wider viewing angle and faster response time.
Generally, there are two main classes of organic light emitting materials of organic electroluminescent display, including small-molecule based light emitting diodes (OLEDs) and polymer light emitting diodes (PLEDs). Although both classes of materials have conjugated chemical structures and high luminescent efficiencies, the differences of molecular weights thereof is huge. Organic molecules of the small-molecule based light emitting diodes (OLEDs) have low molecular weight of about couple hundreds, while organic molecules of the polymer light emitting diodes (PLEDs) have extremely high molecular weight of about ten thousands to couple millions. Also, in the fabrication of the light emitting layer of pixels, the small-molecule based light emitting diodes (OLEDs) material is generally deposited by vacuum thermal evaporation (VTE) through the shadow mask. The polymer light emitting diodes (PLEDs) material is dispersed at pixels, typically by spin coating or inkjet printing, wherein the procedures require no vacuum environment, and the processing equipment is more inexpensive. The polymer light emitting diodes (PLEDs) material is suitable for applying to the devices with large area. The PLEDs material is usually used for the applications of large sized displays. However, the small-molecule based light emitting diodes (OLEDs) material has better result of film formation, which is suitable for applying to the high-end products. The OLEDs material is usually used for the applications of medium or small sized displays.
Take three different colored sub-pixels (R, G, B sub-pixels) to exemplify the evaporation of materials capable of emitting red, green and blue light colors independently. Please refer to FIG. 1A˜FIG. 1C, which illustrate an organic light emitting material evaporated in a vacuum system. The upper and lower portions in the figures depict the top views and the cross-sectional view of the substrates and the related elements on the substrates. A substrate 11 is disposed under a shadow mask 12, and the organic light emitting material 15 is then deposited on the certain areas through the shadow mask 12 by the vapor thermal evaporation (VTE). As shown in FIG. 1B, a shadow mask 12 is disposed on the substrate 11. Typically, the shadow mask is made from the metallic material with low thermal expansion coefficient, and so-called as the metal mask or the fine metal mask (FMM). The shadow mask 12 could be formed by welding a thin metal plate on a metal frame 13, wherein the thin metal plate have a predetermined pattern (such as the openings 12a arranged as an array) formed by electroforming or etching. When one of red, blue and green organic materials is deposited, the pattern of the shadow mask 12 corresponds to the positions of one colored pixels but shields the positions of other two colored pixels. For example, the openings 12a (FIG. 1B) are corresponding to the positions of red pixels, and the evaporation source 14 contains the red organic light emitting material. During evaporation, the red organic light emitting material is deposited through the openings 12a to form the organic light emitting material 15 at the red pixel areas (FIG. 1C). The shadow mask 12 or the substrate is then moved by a high accuracy alignment system to deposit another colored light emitting material.
In the conventional evaporation, each opening of the shadow mask corresponds to a position of one sub-pixel. Accordingly, the resolution of the conventional device is restricted by the processing ability of FMM.
When a display requiring high resolution is fabricated, accuracy of alignment system, tolerance of opening size, clogs and contamination at the openings of the shadow mask would cause considerable effects on the production quality since the size and pitch of pixels are decreased. Currently, the highest resolution of the commercial product, fabricated by aligning one opening of the shadow mask to one sub-pixel in the conventional evaporation, is about 300 PPI.